#include<iostream>

using namespace std;

int main()
{
    int n;
    cin >> n;
    float sum = 0;
    for (int i = 1; i <= n; i++)
    {
        float x;
        cin >> x;
        sum += 1 / x;
    }
    sum /= n;
    sum = 1 / sum;
    printf("%.2f", sum);
    return 0;
}